LEVEL OF THE GLACIER ANNUALLY RENEWED. 
203 
could be desired, that the kind of internal motion necessary for producing the frontal 
dip in the veined structure (which arises from tearing or crushing in sliding in the 
vertical plane*) was correctly foreseen. 
The law of velocities at different points of the axis of a glacier from its origin to 
wards its termination, must evidently depend upon the configuration of each parti- 
cular glacier. It may be constantly increasing from the origin to the extremity, it 
maybe diminishing, or it may have alternations of increase and diminution; and 
upon this circumstance the frequency and magnitude of the crevasses will mainly 
depend. But the regime of the glacier, by which we mean to express the combina- 
tion of circumstances determining its motion, varies from one season of the year to 
another, owing not only to the general influence of heat and cold, but also to the pro- 
gressive communication of that influence to portions of the glacier in successive stages: 
of elevation. Evidently the extremity nearest to the valley will receive the earliest 
and most violent impression of solar heat, whilst the middle and upper regions are 
involved in complete winter. Partial dilatations must take place in spring, partial 
condensations in the decline of the year; as is evident from the consideration 
that temperatures inferior to freezing do not sensibly affect the motion of the ice (see 
above, p. 192) which higher temperatures do, consequently the influence of season 
will be chiefly felt in those parts of the glacier where the temperature of the air sel- 
dom falls in summer to 32°, whilst the more stable motion of the higher part acts as 
a drag or equalizer upon the whole system. The condition of violent distension 
produces crevasses, that of violent compression produces the frontal dip of the veined 
structure, or that share of it which is due to the relative motions in a vertical plane. 
The longitudinal veins will result whether the axis of the glacier be distended or coin- 
pressed. Hence the reason why the frontal dip is difficultly seen in all the middle region 
of a glacier, which like the Mer de Glace, is subject to much extension due to great 
and increasing declivity, and to be well seen must be sought for in the higher parts of 
the glacier, as above Trelaporte, at the foot of the Couvercleq-j and in glaciers subject 
to great compression, as that of La Brenva, the glacier of the Rhone, the Aar, &c. 
Ablation of the Surface . — One phenomenon is most satisfactorily explained by the 
variations of velocity established and illustrated in this paper. The collapsed state 
of the glacier after the hot summer of 1842, and the absolute lowering of its surface 
level by thirty feet in the space of a few months, had struck me as requiring an energy 
altogether extraordinary in kind and degree to restore next spring the level which 
had been lost, in order to allow for an equal ablation the succeeding summer; and 
at first I was disposed to admit so much of the dilatation theory to be true as would 
account for the swelling of the surface in a vertical direction by the freezing during 
winter of the infiltrated water^. Further reflection convinced me however that this 
explanation was insufficient and also not required, and I accordingly concluded 
* Seventh Letter on Glaciers. Appendix to Travels, p. 435. 
t Travels, 2nd edit., p. 167. $ Fourth Letter. Travels, App., p. 415. 
